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doi:10.2204/iodp.proc.308.211.2010

Methods

Triaxial tests were performed at University of Freiburg (Germany), using an ELE Digital Tritest 50 apparatus. The tests performed were consolidated and undrained (CU-test). In addition to sediment strength properties derived from the compression phase, permeability and hydraulic conductivity were determined on the basis of consolidation data. Setup and use of the apparatus are extensively documented in Roller et al. (2003) and Röser (2007). To date, a total of 13 tests (12 successful, 1 failed) were performed to approximately simulate in situ conditions in the overpressured Ursa Basin sediments, and results are shown below. Tests were carried out in constant strain rate mode to large (~20%) axial shortening in order to elucidate the postyield behavior. Material properties, like cohesion, coefficient of friction, and internal angles of friction, can be estimated from these tests. Consolidation of the samples after saturation was used to determine hydraulic conductivity and permeability in a routine akin to oedometer testing. A single triaxial test required ~2 weeks, including supplementary measurements, saturation, consolidation compression, and posttest data analysis. Two, or preferably three, tests were performed at confining pressures ranging from 0.5 to 1.8 MPa on a whole-round sample at different confining pressures. This was achieved by dividing the whole round into two or three aliquots.

The maximum vertical load possible with the ELE Digital Tritest 50 apparatus is 7.5 kN (Roller et al., 2003). The cylindrical specimens were cut from undisturbed whole-round core. Constrained by the size of the cell base, specimen diameter is 35 mm and specimen height chosen was ~70–75 mm (DIN, 1990). Before installation on the cell base, the specimens were wrapped in filter paper, and porous disks were fitted to both ends of the specimen before being inserted into an impermeable rubber hose to avoid contact with the pressure cell fluid. The hose ensures that the specimen has no contact with the water filling the pressure cell. The cell base contains the influx to and drainage off of the cell and the specimen. Outlets and inlets are each equipped with sensors and connected through tubing with hydraulic pumps and/or with the volume change unit and pressure gauges.

Water content and grain density were measured prior to testing to correctly assess saturation pressures. Water content was measured following German Industry Standard DIN 18121 (DIN, 1998) by oven-drying 30–50 g of the sample material at 105°C for 12 h. Oven temperature must not be higher in order to avoid the release of interlayer water in the clay minerals. Grain density was measured following German Industry Standard DIN 18124 (DIN, 1997).

A complete triaxial test comprises three phases: saturation, consolidation, and compression. Saturation of the sample with salt water is achieved by a stepwise increase of cell and pore pressure and is necessary to remove remaining air in the pore water and space (see Roller et al., 2003). Air in the pores corrupts the results of the compression test. To achieve saturation, the cell pressure and the pore pressure are increased simultaneously. We found that saturation of clayey to silty sediments required ~3 days. During the following 16–24 h of consolidation, isotopic equilibrium stress is achieved. During the compression stage, all valves from or to the triaxial cell base are closed to ensure an undrained test. Cell pressure is maintained by the water pumps while the specimen is brought to failure by a piston advancing at constant speed. Axial shortening rate was chosen to be 0.03 mm/min, which is sufficiently low to equilibrate to the confining pressure. Empirical tests (e.g., Röser, 2007) with similar materials have shown this to be the optimum shortening rate for undrained triaxial tests. Tests were terminated after 20%–28% of axial strain. During the test, the following parameters were measured in logarithmic time steps: axial displacement, cell pressure, pore pressure, backpressure (i.e., pore pressure in the sample during the consolidation and shearing stages), and axial force. From the data we calculated Young’s modulus, cohesion, and angle of friction, using the routines outlined by Roller et al. (2003) or Röser (2007).

Permeability and hydraulic conductivity were determined by inserting consolidation data from triaxial testing into Darcy’s law (see Röser, 2007, for details of the calculation procedure). During consolidation, an external pressure is imposed on the specimen, resulting in a pressure difference outside and inside the specimen. The resulting pore water flux out of the specimen through the porous discs at the specimen base and top was monitored by the volume change transducer. The resulting flux data enable the calculation of the total discharge. To define the hydraulic gradient, the length of the pressure drop was chosen to be half the length of the specimen, as the specimen was double drained.

Ring shear tests were performed at the RCOM Institute, University of Bremen (Germany), using a Bromhead RS ring shear apparatus (see Röser, 2007, for description of equipment and analytical procedures). For additional information on ring shear testing, the reader is referred to, for example, Bromhead (1979), Lupini et al. (1981), and Harris and Watson (1997). To simulate high-strain deformation during large movements on slump surfaces, water-saturated remolded sediments were sheared to high strains. It is the principal objective of the ring shear experiments to study the high-strain behavior. The use of remolded samples precludes the study of the effects of initial clay fabrics on strength. This, however, is taken to be of minor importance, as initial fabrics (e.g., degree of shape preferred orientation) in the clays studied is low in marine clays sedimented by suspension fallout from an oxic bottom water column (e.g., Kopf and Behrmann, 1997). Measurements were performed with axial loads ranging from 1 to ~16 MPa at four different rates of shear. A computer program stored all necessary data and directly computed the main shear parameters, such as shear stress, coefficient of friction, and internal angle of friction. Because of the very high clay content, the samples required very long consolidation periods, resulting in ~10 days duration for a single test. Four samples were measured until now, and the results are given below.

The experimental procedure is as follows. Annular remolded soil samples of 5 mm thickness and with inner and outer diameters of 20 and 50 mm, respectively, are confined radially between concentric rings. The samples are compressed vertically between porous bronze loading plates by means of a lever loading system and kept under full seawater saturation during the whole test to ensure the appropriate hydration state for hydrous clay minerals in a marine environment (e.g., Kopf and Brown, 2003). One full test consisted of up to five incremental loading steps (0.962, 1.914, 3.817, 7.624, and 15.237 MPa). Each step was preceded by at least 12 h of consolidation. This is necessary to eliminate effects on the shear strength of the tested material by incompressible pore water to be able to study the influence of mineralogy and granular structure on friction and strength. Following consolidation, shearing was performed at four different velocities: 0.005, 0.014, 0.18, and 1.8 mm/min. Shearing is induced by torsion of the basal plate and the lower platen by a motor and gearbox. The settling of the upper plate during consolidation and shear was recorded by a displacement transducer on the top of the load hanger. Torque transmitted through the sample was recorded by a pair of matched load-recording proving rings bearing on a cross arm. The principal advantage of ring shear testing in comparison to direct shear testing (e.g., Berg, 1971) is the possibility to carry out isochoric simple shear to quasi-infinite strain. In nature, this is the case along the failure plane of a landslide or in a fault zone. Shearing at different shear velocities was used to determine the velocity dependence of friction, giving information about the weakening or strengthening capabilities of the tested material (e.g., Scholz, 1998).

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